[TASK] Improve travis notifications to channels

By default travis notifies on each build when
posting to channels (irc, slack)

We can reduce the number of notifications by only
posting successful builds when it previously failed.

Additionally encrypt the API token for posting to slack.

Releases: 6.3, 6.2, 6.1, 6.0, 4.7, 4.5
Change-Id: I882d34903c972201454e6cc5b9041393e3bd3661
Reviewed-on: https://review.typo3.org/31218
Reviewed-by: Michael Stucki
Tested-by: Michael Stucki